preparing a pretreatment liquid containing water and an aggregating agent;

preparing an ink containing water and a colorant;

applying the pretreatment liquid onto an impermeable base material, heating and drying the pretreatment liquid after applying the pretreatment liquid and before applying the ink, and applying the ink onto a region of the impermeable base material where the pretreatment liquid has been applied; and

drying the ink applied onto the region to obtain an image,

wherein the drying includes blowing of hot air at a temperature of 40° C. or higher and a wind speed of greater than 15 m/s to the ink applied onto the region, and

wherein, when the number of grams of the ink applied per 1 m² in an image area with a density of 100% is defined as X, and the number of grams of the pretreatment liquid applied per 1 m² in the image area with a density of 100% is defined as Y, a viscosity of a kneaded material at 25° C. obtained by adding (6.5Y/X) mg of the pretreatment liquid to 10 g of the ink and defoaming and kneading the mixture at 200 rpm for 5 minutes is in a range of 30 mPa·s to 500 mPa·s, and X is 2.0 or greater.